Optical fibers composed of a core and a clad coated on the core are utilized as a light guide. A core having a high optical transmittance has been made of glass or plastic and the clad has been made of glass or resin having a refractive index lower than that of the core. Flexible plastic-clad optical fibers having a relatively thick core have been developed for indoor or short distance applications. Further, optical fibers having a two-layered clad have also been developed, the outer layer having a refractive index lower than that of the inner layer.
Various thermoplastic resin compositions have been used for optical fibers coatings, but the use of such thermoplastic resins requires a relatively long curing time at a high curing temperature, causing some thermal degradation of the fiber's optical property as well as poor productivity.
To solve such problems, U.S. Pat. No. 5,238,974 discloses a UV-curable cladding composition consisting essentially of a fluorinated monofunctional acrylate composition, a polyfunctional acrylate and a photoinitiator. U.S. Pat. Nos. 5,690,863 and 5,951,920 describe an optical fiber cladding composition comprising a highly fluorinated monofunctional (meth)acrylate monomer with a low refractive index of about 1.37. Further, EP 0 349 126 A2 discloses a cladding composed of an amorphous fluoroplastic resin having a high glass transition temperature. Though such UV curable acrylate-functional formulations have the advantage of fast curing, they are too viscous for die-coating applications. U.S. Pat. No. 5,822,489 provides a photocurable cladding material based on a fluorinated urethane acrylate. However, the fluorinated urethane acrylate oligomer absorbs light at about 1530 nm causing optical loss.
Accordingly, the present inventors have endeavored to develop a novel clad resin composition for optical fiber having improved properties and processability.